A PDA is a computer small and light enough to be carried on the person. It is so small that a full-alphabet keyboard would have keys so small as to be of little use. Accordingly, the PDA includes a removable stylus. The user presses the stylus onto a User Interface (UI). The UI is pressure sensitive to the touch of the stylus, so that the user may enter data. The UI is also a monitor, and thus can display data as well.
The PDA may be combined with a wireless telephone. It is more convenient for the user to have a single device on his belt, rather than having two. Moreover, data may be quickly and accurately transferred between the telephone and PDA. A keypad hinges from the bottom of the PDA. When the keypad is closed, the exterior of the keypad displays the keys which may be pressed to place a telephone call. When closed, the keypad covers some (but not all) of the User Interface. When the keypad is open, the remaining portions of the UI are exposed, and the interior of keypad shows the stylus strokes which may be used to enter data into the UI.
It is important that the power on/off of the telephone be coordinated with the power on/off of the PDA If the telephone is improperly off, a call may be dropped or missed. If the PDA is improperly off, data may be lost. If either or both are improperly on, battery life will be shortened.